1. Field
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
2. Description of the Related Art
Nitride semiconductors typified by GaN, AlN, InN and their mixed crystals are drawing attention as materials for high-power devices and short-wavelength light-emitting devices because of their excellent material characteristics. As a high-power device, a high electron mobility transistor (HEMT) made of a nitride semiconductor has been disclosed in JP-A-2002-359256.
For example, as illustrated in FIG. 7, the gate electrode 40 of an HEMT made of a conventional nitride semiconductor is formed by forming a gate opening in a protection film 35 covering the semiconductor surface between the source electrode 31 and the drain electrode 30 thereof and by filling the lower end of the gate electrode 40 into this gate opening. At this time, the protection film 35 makes contact with the side faces of the lower end of the gate electrode.
After forming the gate electrode, usually, heat treatment is performed to reform the material of the gate electrode, and ultraviolet light irradiation and plasma treatment are performed to densify the protection film. After these treatments, the protection film 35 shrinks and a clearance 90 is formed between the protection film 35 and each of the side faces of the lower end of the gate electrode 40, whereby the semiconductor surface is exposed.
When the HEMT operates, electric field concentration occurs at the lower end of the gate electrode and at the drain-side fringe of the gate electrode, whereby electrons are trapped on the semiconductor surface exposed in this region. The trapped electrons may cause current collapse.